How the gas then ultimately reaches the forming planet, with a size of order \(R_{\mathrm {p}}\sim (0.01\textrm {--}0.001){R_{\textnormal {Hill}}}\) , is an open question.
From angular momentum conservation, and by analogy with objects across a large range of mass scales, a part of the matter likely goes through a CPD (which in its outer regions may be a decretion disc, i.e. exhibit an outflow; [1]}; the general theory of decretion discs is presented in [2]}, [3]}, [4]}) while the rest could fall directly onto the proto-gas-giant with a polar shock (see [5]}, [6]}). However, the fraction of the gas processed through a CPD is unknown, as is
how the gas leaves the CPD to be incorporated into the planet. Obvious possibilities are processes invoked for CTTSs (see review in [7]}), including
boundary-layer accretion (e.g. [2]}, [9]}, [10]}; see brief review in [11]}),
or
magnetospheric accretion
([12]}, [13]}, [14]}; see also [15]}).
As in the stellar context (see reviews in [16]}, [17]}), which of these two mechanisms dominates will depend on the magnetic field strength of the forming planet and the coupling of the gas to the magnetic field.
In the boundary-layer accretion (BLA) scenario there is no accretion shock on the planet surface. Therefore, we do not treat BLA here; in this case the observed H \(\alpha \)  would need to come from a shock on the CPD [18]}.
